Electrochemical process for polishing tantalum



Patented Apr. 5, 1949 ELECTROCHEMICAL PROCESS FOR POLISHING TANTALUM John Frederick Gall, Upper Darby, and Henry Conrad Miller, Philadelphia, Pa., assignors to The Pennsylvania Salt Manufacturing Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application March 15, 1944, Serial No. 526,656

2 Claims. 1

This invention relates to electropolishing of metals and electrolytes employed in that operation. More particularly the invention comprises a method of electropolishing tantalum and an electrolyte comprising hydrofluoric acid and a strong acid such as hydrochloric or sulfuric acid.

It is known that tantalum metal exhibits valve action. That is, when a current is passed through the metal, as an electrode submerged in an electrolyte, and in such a direction as to make the metal anodic, a resistance is created causing the flow of current to cease. When this occurs, even the slight degree of anodic solution necessary for electropolishing cannot take place, and it is this phenomenon which has heretofore prevented the electropolishing of tantalum.

An object of this invention is to provide a method of polishing tantalum metal wherein the valve action of the metal is overcome.

Another object of the invention is to provide an electrolyte enabling the contemplated method to be carried out.

Certain other objects and advantages are apparent from a consideration of this specification and claims.

According to the invention, the tantalum metal to be polished is made anodic in an aqueous solution, containing hydrofluoric acid together with hydrochloric acid and/or sulfuric acid of a certain, predetermined, critical composition and cencentration, thereby obtaining, upon passage of current at a certain critical anodic current density, a dissolution of the surface irregularities of the metal giving it a fine high polish.

The composition and concentration of the electrolytic bath employed will preferably be:

Percent Hydrofluoric acid 2- 7 Hydrochloric acid 32-38 the total combined acid concentration being between 34% and 45%, the balance being substantially water, in the case hydrochloric acid is employed; and

Percent Hydrofluoric acid 2- 7 Sulfuric acid 75 98 the total combined acid concentration being between 77% and 100%, the balance being substantially water, when sulfuric acid is employed. The percentage figures are by weight.

A current density of from 40 to 160 milliamperes per square centimeter has been found to avoid valve action, employing an electrolytic bath having a composition and concentration as above stated.

A substantial departure from the conditions of the invention will result in unsatisfactory polishing. The metal may dissolve too rapidly with excessive loss of material and production of an etched surface or it may valve, that is, the passage of current may cease and the surface of the metal remain unchanged or the metal may behave erratically and a periodic pulsation of current, accompanied by the formation of flaky material dropping from the surface, may result.

As specific examples of the invention yielding good results, the following are cited:

Example 1 Tantalum is made anodic at a current density of 67 milliamperes per square centimeter in an aqueous solution of hydrofluoric and hydrochloric acids, which is 4%, by weight, hydrogen fluoride and 34%, by weight, hydrogen chloride. The operation is conducted at room temperature. In about 10 minutes the surface has developed a high mirror-like finish.

Example 2 Tantalum is made anodic with a current density of 40 milliamperes per square centimeter in an aqueous solution of hydrofluoric and sulfuric acids, which is 4%, by weight, hydrogen fluoride and 89%, by weight, sulfuric acid. The operation is conducted at room temperature. In this case polishing will result after about 20 minutes of electrolysis.

The cited examples are in no way restrictive of the invention, its essence being the discovery that tantalum metal can be electropolished employing the electrolytes, set forth, at the indicated ranges of concentrations, and the current densities, also as indicated.

We claim:

1. The electropolishing of metallic tantalum by making it anodic in an electrolyte, containing 2%-'7% by weight of hydrogen fluoride and '75 98% by weight of sulfuric acid, the combined acid concentration being between 77% and 100%, the balance being substantially water, and passing current at an anodic current densit of from 40 to milliamperes per square centimeter until the metal surface has developed a high, mirror-like polish.

2. The electropolishing of metallic tantalum by making it anodic at a current density of 40 milliamperes per square centimeter in an aqueous solution of hydrofluoric and sulfuric REFERENCES CITED.

The following references are of record in the 1 file of this patent:

UNITED STATES PATENTS Number Name Date 2,115,005 Blaut et a1. Apr. 26, 1938 15 2,153,060 Guthrie Apr. 4,1939

FOREIGN PATENTS Number Country Date 21,950 Great Britain 1891 5 530,041 I Great Britain Dec. 4, 1940 OTHER REFERENCES Transactions of The Electrochemical Society, vol. 78 (1940), pages 265 through 272.

Comprehensive Treatise on Inorganic Chemistry, by Mellor, vol. 9 (1929), page 891. 

